diff --git a/CHANGELOG.md b/CHANGELOG.md
index de21604..fdbc7db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,15 @@
 ## Changelog
 
+### v0.66.1 — fix USERDATA_PATH on first deploy (2026-06-14)
+
+The initial deploy path (`DeployStack` → `composeExecWithEnv`) builds its compose env from the deploy
+values, not from app.yaml via `stackEnv` — so v0.66.0 injected `USERDATA_PATH` only on start/redeploy,
+NOT on the FIRST deploy. A freshly-deployed app resolved `${USERDATA_PATH}` to `""` and Docker bound a
+bogus root-owned dir at the container root (e.g. `/media/movies`) instead of `<drive>/userdata/...`
+(found live: radarr's media mount was `0:0 755` at the container root). Fix: a shared `withUserdataPath`
+injector used by BOTH `stackEnv` and `composeExecWithEnv`. Regression test asserts injection on/off by
+HDD_PATH presence.
+
 ### v0.66.0 — userdata layout + shared-storage ownership convention (2026-06-14)
 
 Customer-facing `userdata/` tree (sibling of appdata/backups under each drive's felhom-data namespace)
diff --git a/controller/internal/stacks/deploy.go b/controller/internal/stacks/deploy.go
index e8d12e4..ae45fa6 100644
--- a/controller/internal/stacks/deploy.go
+++ b/controller/internal/stacks/deploy.go
@@ -13,6 +13,7 @@ import (
 	"strings"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/crypto"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/system"
 	"gopkg.in/yaml.v3"
@@ -513,16 +514,31 @@ func (m *Manager) RedeployFromEnv(name string, env map[string]string) error {
 	return m.RefreshStatus()
 }
 
-// composeExecWithEnv runs a compose command with custom env vars injected.
+// composeExecWithEnv runs a compose command with custom env vars injected. Used by the initial deploy
+// path (DeployStack), which builds env from the deploy values rather than from app.yaml via stackEnv —
+// so USERDATA_PATH must be injected here too (mirrors stackEnv), else the FIRST deploy resolves
+// ${USERDATA_PATH} to "" and binds a bogus root-owned dir at the container root.
 func (m *Manager) composeExecWithEnv(dir string, env map[string]string, args ...string) (string, error) {
 	cmdEnv := os.Environ()
 	for k, v := range env {
 		cmdEnv = append(cmdEnv, fmt.Sprintf("%s=%s", k, v))
 	}
 	cmdEnv = append(cmdEnv, fmt.Sprintf("DOMAIN=%s", m.cfg.Customer.Domain))
+	cmdEnv = withUserdataPath(cmdEnv, env["HDD_PATH"])
 	return m.composeExecCustomEnv(dir, cmdEnv, args...)
 }
 
+// withUserdataPath appends USERDATA_PATH=<hdd>/userdata to a "K=V" env slice when hdd is non-empty.
+// Shared by BOTH compose-env builders (stackEnv for start/redeploy, composeExecWithEnv for the initial
+// deploy) so ${USERDATA_PATH} always resolves — the initial-deploy path missing it bound a bogus
+// root-owned dir at the container root.
+func withUserdataPath(cmdEnv []string, hdd string) []string {
+	if hdd != "" {
+		cmdEnv = append(cmdEnv, "USERDATA_PATH="+appbackup.UserdataDir(hdd))
+	}
+	return cmdEnv
+}
+
 // GetDeployFields returns the deployment fields for a stack (for the deploy form).
 func (m *Manager) GetDeployFields(name string) (*Metadata, *AppConfig, error) {
 	stack, ok := m.GetStack(name)
diff --git a/controller/internal/stacks/manager.go b/controller/internal/stacks/manager.go
index 8ae336e..efb4d2c 100644
--- a/controller/internal/stacks/manager.go
+++ b/controller/internal/stacks/manager.go
@@ -860,9 +860,7 @@ func (m *Manager) stackEnv(stackDir string) []string {
 		// Inject USERDATA_PATH = <namespace root>/userdata alongside HDD_PATH (v0.66.0). HDD_PATH IS
 		// the namespace root (the chosen StoragePath: a Model-A user drive's mount, or the SSD's
 		// felhom-data dir), so the catalog's ${USERDATA_PATH}/... mounts resolve under userdata/.
-		if hdd := appCfg.Env["HDD_PATH"]; hdd != "" {
-			env = append(env, fmt.Sprintf("USERDATA_PATH=%s", appbackup.UserdataDir(hdd)))
-		}
+		env = withUserdataPath(env, appCfg.Env["HDD_PATH"])
 	}
 
 	return env
diff --git a/controller/internal/stacks/userdata_belt_test.go b/controller/internal/stacks/userdata_belt_test.go
index 5bb3043..0bf7c55 100644
--- a/controller/internal/stacks/userdata_belt_test.go
+++ b/controller/internal/stacks/userdata_belt_test.go
@@ -71,3 +71,26 @@ func TestEnsureUserdataMounts_CreatesBeltDirs(t *testing.T) {
 	}
 	_ = appbackup.SharedContentGID // keep import referenced cross-platform
 }
+
+// TestWithUserdataPath: the shared injector adds USERDATA_PATH=<hdd>/userdata when HDD_PATH is set, and
+// adds nothing when it's empty. Regression for the initial-deploy bug where ${USERDATA_PATH} resolved
+// to "" and bound a bogus root-owned dir at the container root.
+func TestWithUserdataPath(t *testing.T) {
+	got := withUserdataPath([]string{"DOMAIN=x"}, "/mnt/felhom-usb")
+	want := "USERDATA_PATH=" + appbackup.UserdataDir("/mnt/felhom-usb")
+	found := false
+	for _, e := range got {
+		if e == want {
+			found = true
+		}
+	}
+	if !found {
+		t.Errorf("USERDATA_PATH not injected: got %v, want %q", got, want)
+	}
+	// companion: empty HDD_PATH → no USERDATA_PATH at all
+	for _, e := range withUserdataPath([]string{"DOMAIN=x"}, "") {
+		if len(e) >= 13 && e[:13] == "USERDATA_PATH" {
+			t.Errorf("USERDATA_PATH must NOT be set when HDD_PATH is empty: %q", e)
+		}
+	}
+}